This invention relates to transmission lines, and more particularly to active terminator buffers with positive feedback.
Performance of electronic systems can be improved by increasing clock rates. However, as clock rates rise above 100 MHz, metal traces on circuit boards behave as transmission lines, exhibiting reflections, undershoot, overshoot, and ringing that distort signals.
Expandability of systems also can create problems. For example, a personal computer may be shipped with a base memory having 18 memory chips driven by a clock line. The clock driver is able to drive the 18 memory chips without serious signal distortion. However, when the user later expands the memory to 36 memory chips the clock driver is unable to drive the doubled load without serious signal distortion.
FIG. 1 shows a transmission line driving a load. Driver 10 drives one end of transmission line 12, which can be a metal trace on a circuit board, or a cable or other connection to another board or component, or some other combination. Load 14 at the far end of transmission line 12 is being driven, and can represent several chip inputs such as pins on memory chips.
When driver 10 drives its output from low to high, the near end of transmission line 12 is driven high while the far end remains low. A wave front travels down transmission line 12 until it reaches load 14 at the far end, driving it high. However, the wave front can reflect off load 14 and a reflected wave travel backwards along transmission line 12 to driver 10. Further reflections can then occur.
FIG. 2 shows an active terminator for a transmission line. Termination resistors are often added to suppress reflections. For example, a series resistor can be added between the output of driver 10 and the near end of transmission line 12, and a resistor can be added between the far end of transmission line 12 and ground or some other fixed voltage.
Terminating resistors can also be actively enabled and disabled by software, hardware, or even in response to a wave front traveling down the transmission line. Terminating resistor 18 is connected to ground when switch 19 is closed in response to an enable signal, and likewise pull-up resistor 16 connects the far end of transmission line 12 to a power-supply voltage when an enable closes switch 17.
Switches 17, 19 can have the same enable signal or different enable signals, and these signal can be static or stable for long periods of time that include many high and low transitions of transmission line 12, or can be dynamic and change for each high or low-going transition. Such active terminators often act as negative feedback to suppress signal reflections.
What is desired is an active terminator for a transmission line that uses positive feedback rather than negative feedback. An active terminator that isolates the load from the transmission line for parts of a cycle is desired. A more sophisticated active terminating buffer is desired.